1. Field of the Invention
The present invention relates generally to electrically erasable, programmable read-only memory structures.
2. Description of the Related Art
Electrically erasable programmable read-only memory (EEPROM) is a non-volatile memory structure often used in electronic systems for storage of small amounts of data (e.g., calibration tables and device parameters) that must be saved when system power is removed to therefore be accessible when power is restored. A exemplary EEPROM memory cell includes a storage transistor and an associated second transistor which is used to program the storage transistor. The storage transistor is generally a metal-oxide-semiconductor (MOS) transistor in which a floating gate is positioned between the control gate and the semiconductor channel. With no charge programmed on the floating gate, the transistor is in an off state in which current is not conducted. When a charge is inserted into the floating gate, the transistor is biased into a stable on state in which current is conducted.
If conventional EEPROM's are used in integrated circuits in which a substrate voltage level differs from the circuit ground, they must be isolated from the substrate to insure the reliability of their stored data. An exemplary isolation structure is formed by surrounding the EEPROM cell with a semiconductor material of a type opposite that of the substrate. A semiconductor junction is thus established about the cell and when this junction is reverse biased it provides considerable isolation. A different isolation structure is formed by etching a trench in the substrate and filling the trench with a dielectric (e.g., silicon dioxide).
Such isolation structures, however, are generally too expensive to be used in low cost CMOS integrated circuits that are intended for use in markets in which cost is a controlling parameter (e.g., consumer markets).